In television receivers having run and standby modes and using switched mode power supplies, an unregulated DC voltage is derived from a bridge rectifier coupled to a mains source. The unregulated DC voltage is then supplied to a primary winding of a power transformer that isolates the hot ground side of the chassis from the cold ground side. A standby regulator, such as the SGS Thomson Microelectronics TEA2260 or TEA2261, is located on the hot side and controls a switching means such as a chopper transistor to apply current pulses of variable pulse width and amplitude to the primary winding. Regulated voltages are developed on secondary windings of the power transformer. The standby regulator is powered from the unregulated B+ voltage when the television receiver is first coupled to the mains, and thereafter provides the regulating action in a standby mode of operation until the apparatus is switched into the run mode, whereupon the standby regulator is controlled by a run regulator located on the hot ground side.
Loads other than the standby regulator, including the standby loads and run mode loads such as the horizontal deflection circuit, are coupled to secondary windings of the power transformer. The run mode loads are inactive in the standby mode. Run mode loads may be coupled to regulated voltages, but switched off in the standby mode by a signal from a controller such as a microprocessor responsive to a remote control. Alternatively, run mode loads may be coupled to secondary windings of the horizontal flyback transformer, and are unpowered in the standby mode because, although the flyback transformer is coupled to a regulated B+ voltage, no voltages are generated in the secondary windings of the flyback transformer in the absence of horizontal scanning.
The standby regulator operates at a stable frequency and has a pulse width modulator that regulates voltages on the secondary windings of the power transformer by adjusting the pulse width or duty cycle of the voltage pulses applied to the chopper transistor. In the standby mode, the standby regulator is made to free run at its local oscillator frequency, and regulates by sensing the voltage on a hot ground secondary winding of the power transformer. The sensed voltage is coupled to an input of an error amplifier having a second input coupled to a voltage reference. Regulation of the B+ scan supply voltage occurs indirectly, because the voltages on all the secondary windings go up or down together.
In the run mode, the standby regulator becomes the slave of a run regulator on the cold ground side. The run regulator may be, for example, the SGS Thomson Microelectronics TEA5170. The standby regulator continues to switch the chopper transistor on the primary winding of the power transformer for producing regulated voltages on the secondaries, but the standby regulator is driven in the run mode by a pulse width modulation signal generated by the run regulator. The run regulator is synchronized with the horizontal deflection by e.g. horizontal flyback pulses. The regulated B+ voltage for the horizontal deflection circuit is fed back to an error amplifier in the run regulator and compared to a reference for generating the pulse width modulation signal that is coupled back to the standby regulator. The pulse output of the run regulator is used in the run mode to supplant the output of pulse width modulation circuitry in the standby regulator.
When the feedback to whichever of the regulators that is controlling, differs from the applicable reference level, the regulator's pulse width modulation circuit changes the pulse width or duty cycle of output pulses, to vary the power through the power transformer and regulate the output voltage. In order to avoid generating too much current in the chopper transistor and power transformer, a number of safety features are built into the run and standby regulators and into the controller that switches between run and standby modes.
A current sensing resistor is coupled in series with the chopper. This resistor provides a voltage representing the current level in the chopper and in the primary winding to a shutdown circuit in the standby regulator. The shutdown circuit of the TEA2260/61 has two threshold levels. If current passes a lower threshold, the on-pulse of the pulse width modulated output is immediately terminated but is redeveloped at the next cycle of operation. If current passes a higher threshold, the standby regulator shuts down and will not restart until VCC is removed, e.g. by pulling the mains plug.
The standby and run regulators each include other protective features and sensing circuits arranged to disable operation when their VCC power supply voltages goes above or below internally defined reference levels.
Each regulator also has circuitry to limit the maximum duty cycle of its output by substituting a fixed duty cycle output when the feedback loop attempts to drive the output duty cycle beyond a predetermined limit.
These and other protective features generally handle the load increases that occur with startup and when switching from standby to run, when the pulse width modulators would otherwise seek to increase the duty cycle of the pulse output to bring the output voltages up to their references, with possible damage to the chopper. However, it remains possible that under certain conditions, particularly in the run mode, that changes in loading can cause the duty cycle to be varied rapidly to a point where the chopper transistor is overstressed.
The maximum duty cycle of the TEA5170 run mode regulator is, for example, nominally 78%. The normal duty cycle may be relatively high when the television receiver is a high powered e.g., a large screen set operating under high load conditions, such as high picture brightness and high audio loading. The duty cycle also will be relatively high if the AC mains voltage is relatively low.
These situations may cause the duty cycle of the run regulator to occasionally reach its upper limit. Where operation is such that loading in the run mode varies, for example when the picture includes a change from very dark to very light areas, the run regulator also may increase the duty cycle to its upper limit. This could result in a potential overstressed operating condition.
Another potential problem is encountered when AC mains voltage is lost when operating in the run mode. This can occur in the event of a power outage or when the television receiver is unplugged while operating in the run mode. As the mains voltage is falling, the run regulator increases the duty cycle of the chopper in an attempt to maintain the nominal regulated B+ output voltage. The VCC sensing and maximum current protective features of the run and standby regulators may be marginal or ineffective in preventing overstress conditions in the chopper due to excessive duty cycle operation or fluctuations.
The television receiver microprocessor which generates the ON/OFF signal for switching between run and standby modes is typically supplied with a power reset circuit which detects loss of operating supplies. Once such loss is detected on, e.g., an AC line isolated supply, the microprocessor generates the OFF state of the ON/OFF signal and power to the run regulator is removed. Because power loss is sensed from a secondary winding, activation of the reset circuit may come too late to avoid overstressing the chopper transistor in an AC power loss environment.